John Howard will act to block the move in the nation's capital to allow gays and lesbians to celebrate civil unions, on the grounds it is an attack on the institution of marriage.

The Prime Minister confirmed yesterday that federal cabinet had agreed to overturn the ACT legislation that for the first time grants same-sex couples the right to celebrate civil unions.

It was not clear last night whether couples would still be able to celebrate civil unions before the legislation is disallowed on August 1.

"The legislation by its own admission is an attempt to equate civil unions with marriage," Mr Howard said. "We don't find that acceptable."

The dramatic decision to overturn the civil union laws could be tested in a divisive vote in federal parliament if an MP or senator moves to disallow the Howard Government's decision.

The commonwealth will intervene under section 35 of the ACT Self-Government Act, which states the Governor-General, on the advice of the executive council, may disallow an ACT enactment within six months.

Despite claims the legislation would allow teenagers to celebrate civil unions at a younger age than heterosexuals are allowed to marry, Attorney-General Philip Ruddock said the fundamental concern was the attempt to equate civil unions with marriage. He accused the ACT of being "deliberately provocative" with the legislation.

"The Marriage Act makes it clear that marriage is a union between a man and a woman to the exclusion of all others," Mr Ruddock said.

"That definition reflects the traditional understanding and was supported bipartisanly in 2004. The ACT's Civil Unions Act creates a statutory scheme in recognition of relationships which bear a striking resemblance to the commonwealth's regulation of marriage."

After speaking to Mr Howard last night, ACT Liberal senator Gary Humphries said he had deep concerns about the intervention in the autonomy of the territory to make laws. "Having been granted self-government, the ACT ought to be able to make decisions without the federal parliament pushing it around and telling it what it can and cannot do," he said.

"Although I may not agree with the legislation, I don't think it's right for the Government to tell the ACT what it can do in a way it wouldn't be able to with the states."

ACT Attorney-General Simon Corbell said he was "very angry" about the decision.

"This is not about the institution of marriage, this is about the raw politics of John Howard and Philip Ruddock's conservative social agenda," he said.

"I think thousands of gay and lesbian couples in meaningful, committed, loving relationships will be angry and insulted that John Howard is seeking to impose his moral agenda on their relationships.

"I will be lobbying federal members of parliament to seek to disallow it. There should be a debate in the federal parliament on this - that's what happened in relation to euthanasia."

Queensland Liberal MP Warren Entsch, who has proposed a private member's bill to recognise same-sex relationships and remove legal discrimination, said he had "mixed feelings" about the ACT legislation.

"I am hoping still they can be the first ones to introduce a civil union bill," he said.

"I am hoping there can be further adjustment. I have no hesitation in continuing my push to end legal discrimination. I hope in the near future South Australia and Victoria will bring on similar legislation. Tasmania was able to get their legislation through without any hassles."

Tasmanian gay activist Rodney Croome said same-sex and heterosexual couples had been able to sign a relationship register since 2004 that granted their relationship the same protection as marriage or de facto status.

"It has proved very important in allowing not only same-sex couples but other couples to provide a sound legal basis, particularly if they want to start a family," he said.

i stand by my original beliefs on the issue. until gays/lesbians completely dissociate themselves from the word "marriage" and leave that as is, no change will come. there needs to be a strong push for civil unions, entitling same sex couples to the same rights as heterosexual couples. the very words "gay marriage" when used together are simply hurting the cause imo, and won't make any positive change.

I don't understand your point. The ACT legislation is for civil unions and yet Howard is still preventing it. I (begrudgingly) accept that marriage has historical connections with the christian faith where it is dictated as between a man and woman and this is why same-sex marriages won't be accepted, however the civial union is to acknowledge couples and give equal rights to them. I don't see how that affects the 'institution of marriage'.